tetrachord

The tetrachord has four sound sources – four basic microphones are fixed on a transparent plastic material stretched in a rectangle. The plastic foil hangs on springs, causing them to vibrate.
A little engine placed in the center of the object with four arms which are effecting the microphone by rubbing the foil. The four piezo pickups are amplified to four loudspeakers, which are arranged in the same constellation in space as the pickups on the object.